This invention relates to the field of battery charge conservation and more particularly to conserving battery charge during the alert cycle of a selective call radio paging receiver.
Selective call radio paging receivers (commonly called "pagers") have traditionally alerted the user to an incoming message by soundinq an audible alert tone. Although other alerting devices have been previously available, the recent market trend has been to combine several alerting devices in one pager and to operate them concurrently. Such alerting devices include vibrators, automatic back-lighting for liquid crystal displays (LCD's), light emitting diodes (LED's), and alert tone generators with either escalating volume or traditional manual volume adjustment.
One problem with such alerting devices is that they consume large amounts of battery charge during their brief period of activation. Thus, combining several alerting devices in one pager and operating these devices concurrently can rapidly deplete a battery.
A particularly troublesome problem, however, occurs in memory display pagers. Memory display pagers can store a plurality of received messages in the pager's random access memory (RAM). This memory typically draws very little current from the battery and messages can be stored there for many hours, even though the battery may be near depletion. But, random access memories are volatile and the messages stored in the memory can be lost if the battery voltage drops below the minimum voltage necessary for information retention.
Ordinarily, the battery is capable of supplying the voltage and current necessary to operate all the circuits in the pager, including multiple alert devices. As the battery nears depletion, however, its voltage drops and its internal resistance increases. Thus, if several alerting devices are concurrently activated, a large load is placed on the battery and its voltage drops below the minimum voltage required for information retention and the stored messages are lost.
If this failure mode, which is sometimes referred to as catastrophic failure, could be prevented, the battery could operate the non-alerting circuits in the pager for several additional hours, even though the battery is near depletion. Thus, previously received messages would still be retained in the memory and new messages could be received during this extended time period.